C++11 value-initialization with defaulted default constructor

Question

In the following example:

#include <iostream>

struct A {
    int z;
    A(std::string) {}
    A() = default;
};

int main() {
    char buf[1000];
    std::fill(buf, buf + 1000, 'x');
    auto a = new(buf) A{};
    std::cerr << "A.z: " << a->z << std::endl;
}

Compiled with GCC 4.8 outputs zero (same behavior with Clang 3.4). This seems to indicate that a is being zero-initialized before the default constructor is called.

But according to value-initialization rules on cppreference.com, the object should not be initialized before the default constructor invocation. Class A qualifies for bullet point #1 under C++11:

1) If T is a class type with at least one user-provided constructor of any kind, the default constructor is called.

Another perhaps useful data point is that if we replace A() = default with A() {} in the above example, no zero-initialization happens as expected. This seems to indicate that bullet point #2 of value initialization is being followed in the initial example which would be wrong:

2) If T is an non-union class type without any user-provided constructors, then the object is zero-initialized and then the implicitly-declared default constructor is called (unless it's trivial)

Casey Casey · Accepted Answer · 2014-06-04T17:46:22

The wording that you cite for value initialization in C++11 was considered defective, see Core Working Group DR 1368 and the resolution in Core Working Group DR 1301 that changed the wording (showing additions with bold and deletions with ~~strike through~~):

To value-initialize an object of type T means:

if T is a (possibly cv-qualified) class type (Clause 9 [class]) with either no default constructor (12.1 [class.ctor]) or a default constructor that is user-provided or deleted ~~constructor (12.1 [class.ctor])~~, then the object is default-initialized ~~default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor)~~;

if T is a (possibly cv-qualified) non-union class type without a user-provided or deleted default constructor, then the object is zero-initialized and, if ~~T's implicitly-declared default constructor is~~ T has a non-trivial default constructor, ~~that constructor is called.~~ default-initialized;

Some compilers - I believe GCC 4.8 and clang since 3.3ish - have already implemented the resolution of DR 1301.

C++11 value-initialization with defaulted default constructor

1 Answers